#include<fstream.h>
#include<iomanip.h>
#include<string.h>
#include<stdlib.h>
#include<stdio.h>
int main()
{ long y,n,n1,i,j,nr1=0,nr=0,k,x;float numar,numar1;
char *ptr,s[1000],s1[1000],c;
ifstream f("cifre.in");
f>>s>>s1>>c>>k;
f.close();
n=strlen(s);
numar=atol(s);
numar1=atol(s1);
strcpy(s1,s);
while(numar<=numar1)
{
  nr1=0;
 //ltoa(numar,s,10);
 sprintf(s, "%f", numar);
ptr=strchr(s,c);
n=strlen(s);
 while(ptr!=NULL)
   {
   nr1++;
   s[n-strlen(ptr)]='#';
   ptr=strchr(s,c);
   if(nr1==k) {nr++; break;}
   }

   numar++;
}
ofstream g("cifre.out");
numar=atol(s1);
numar=numar1-numar+1;
g<<setiosflags(ios::showpoint)<<setprecision(4)<<(float)nr/numar<<"\n";
g.close();
return 0;
}